Lithium ion secondary batteries may be classified as lithium ion batteries using liquid electrolytes and lithium ion polymer batteries using polymer solid electrolytes, depending on the types of electrolytes used therein.
In general, lithium ion secondary batteries using liquid electrolytes may be welded and sealed using a cylindrical or an angular metal can as a container, to be used. Since can-type secondary batteries using such a metal can as a container may have fixed forms, they may be disadvantageous in terms of restricting the designs of electrical products using such batteries as power sources, and reductions in the volumes of such electrical products may not be facilitated. Thus, pouch type secondary batteries formed by disposing and sealing two electrodes, a separation film, and an electrolyte in a pouch formed of a film have been developed and used.
A typical pouch for a lithium ion polymer battery has a multilayer structure formed by sequentially stacking a polyolefin layer, an aluminum layer, and a nylon layer. The polyolefin layer, an internal layer, has heat adhesion properties to function as a sealing member. The aluminum layer, a metal layer, provides mechanical strength and functions as a barrier layer against moisture and oxygen. The nylon layer, an external layer, functions as a base material and a protective layer.
Recently, rechargeable secondary batteries have been widely used as energy sources for wireless devices. In addition, secondary batteries have been prominent as power sources for electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and the like, such vehicles being intended to resolve atmospheric pollution and the like, due to existing gasoline vehicles, diesel vehicles and the like.
Further, in accordance with trends for the lightening and miniaturization of devices, secondary batteries used in the devices have been required to be light and to have thin structures. However, in pouch type exterior members according to the related art, in order to increase the safety of a secondary battery against the occurrence of a swelling phenomenon or gas during charging and discharging cycles of the secondary battery, a width of a sealing part may be relatively large. However, in a case in which the width of the sealing part is large, a battery module and a battery pack receiving the secondary battery may be enlarged, such that sizes of devices receiving the battery module and pack may also be increased.